The present invention relates to sputtering targets and other metal articles as well as methods of making the same. More particularly, the present invention relates to methods for forming powder metallurgy sputtering targets and other metallurgical articles made from valve metal materials.
Sputtering targets are used for many purposes, including producing thin films of metals or compounds. In a sputtering process, a source material is bombarded with plasma ions that dislodge or eject atoms from the surface of a sputter target. The ejected atoms are deposited atop a substrate to form a film coating that is typically several atomic layers thick.
Sputtering targets can be made from valve metal materials. Valve metals generally include tantalum, niobium, and alloys thereof, and also may include metals of Groups IVB, VB, and VIB and alloys thereof. Valve metals are described, for example, by Diggle, in “Oxides and Oxide Films”, Vol. 1, pages 94-95, 1972, Marcel Dekker, Inc., New York, incorporated in its entirety by reference herein.
Semiconductor technology is forecast to be the largest market for tantalum sputtering targets. Semiconductors are the building blocks of a class of microelectronic devices that include microprocessors found in mainframe computers, work stations, and PCs, digital signal processors for cell phones and telecommunication equipment, and application-specific integrated circuits used in digital organizers, cameras, and electronic appliances.
Driven by continuous reductions in costs, device size, and improved performance, copper is replacing aluminum for use as interconnects in next generation semiconductors. To prevent the copper of the interconnects from migrating through the semiconductor device and “poisoning” the transistors and other electronics, a diffusion barrier is commonly interposed between the interconnects and the device. Tantalum (Ta) and tantalum nitride (TaN), which is typically produced by the reactive sputtering of a tantalum target in the presence of nitrogen, are commonly-used barrier materials for copper interconnects. By way of example, microprocessors operating at clock speeds in excess of 1000 MHz, such as AMD's Althon and Intel's Pentium 4, as well as IBM's I STAR and P-750 processors found in modern mainframe systems, each use copper interconnects along with a tantalum diffusion barrier layer.
Films having uniform chemistry and thickness are preferred for diffusion barrier applications. To obtain uniform chemistry and thickness, it is preferable to sputter a target having certain desirable properties, including, high purity, a fine grain size, and a homogeneous texture void of strong (001) texture bands. Commonly, tantalum materials produced from ingot metallurgy (ingot-met) techniques as described, for example, in U.S. Pat. No. 6,348,113 (Michaluk et al.), which is incorporated in its entirety by reference herein, are specified for sputtering applications. Ingot-met tantalum material may produce the purity levels and maximum grain size desirable for diffusion barrier applications. However, by nature, it is difficult to refine and control the grain size and texture homogeneity in high purity, unalloyed and undoped metallic materials. As such, the minimum average recrystallized grain size attainable in wrought high purity ingot-met tantalum targets may be about 10 microns. In addition, ingot-met tantalum targets may also exhibit textural banding and consequently may produce sputtered films of highly variable thicknesses.
Powder metallurgy (powder-met) techniques offer an alternative method of manufacturing tantalum material and tantalum sputtering targets. Proper processing can produce powder-met tantalum sputtering targets having a finer grain size than that attainable in ingot-met tantalum targets. The higher amounts of interstitial impurities inherent in the powder-met materials increases the work hardening rate, and hence the rate of new dislocation line length generation and subsequent recrystallization response during annealing, by behaving like a dispersion of fine particles within the matrix. For this reason, a smaller, more homogeneous grain structure is achieved in commercially produced powder-met tantalum thin gauge strip and wire than that which is attainable in ingot-met tantalum products of similar gauge.
The (isostatic) consolidation of metal powders is a viable and established means of producing certain metal articles having a random and homogeneous texture. The combination of fine grain size having a random distribution of crystal orientations promote the uniformity of work (e.g., homogeneous strain hardending of all grains) during subsequent deformation processing of powder-met tantalum sputtering targets, thus avoiding the formation of sharp texture bands in powder-met sputtering targets. The powder-met tantalum sputtering targets are expected to deposit films having exceptional thickness uniformity.
Commercially available tantalum powder, however, contains unacceptably high levels of oxygen for use in diffusion barrier applications. Under ambient conditions, tantalum metal has a passive coating, e.g., such as approximately 1 nm or less to 3 nm or more thick oxide film that is comprised of tantalum oxide and absorbed oxygen gas (L. A. Rozenberg and S. V. Shtel'makl, “State of Oxygen in Tantalum Powders,” Ivestiya Akademii Naut SSSR. Metally, (4) 1985, pp. 163, incorporated in its entirety by reference herein). Commercial tantalum powder that is deoxidized and then exposed to oxygen to reform a passive oxide coating will still typically contain more than 100 ppm oxygen. Preferably, the oxygen content of tantalum sputtering targets is limited to 100 ppm or less. Excessive oxygen in the sputtering target can lead to the creation of tantalum-oxide within the deposited tantalum nitride barrier layer and a subsequent undesirable increase in the RC delay in the interconnect line.
Accordingly, methods for forming low-oxygen metal powder, and sputtering targets or other metal articles produced from the metal powder, are needed for depositing high-integrity films via reactive sputtering.